MHC class I-like MILL molecules are beta2-microglobulin-associated, GPI-anchored glycoproteins that do not require TAP for cell surface expression.

MILL (MHC class I-like located near the leukocyte receptor complex) is a family of MHC class I-like molecules encoded outside the MHC, which displays the highest sequence similarity to human MICA/B molecules among known class I molecules. In the present study, we show that the two members of the mouse MILL family, MILL1 and MILL2, are GPI-anchored glycoproteins associated with beta2-microglobulin (beta2m) and that cell surface expression of MILL1 or MILL2 does not require functional TAP molecules. MILL1 and MILL2 molecules expressed in bacteria could be refolded in the presence of beta2m, without adding any peptides. Hence, neither MILL1 nor MILL2 is likely to be involved in the presentation of peptides. Immunohistochemical analysis revealed that MILL1 is expressed in a subpopulation of thymic medullary epithelial cells and a restricted region of inner root sheaths in hair follicles. The present study provides additional evidence that MILL is a class I family distinct from MICA/B.

Killer cell lectin-like receptor G1 binds three members of the classical cadherin family to inhibit NK cell cytotoxicity.

Killer cell lectin-like receptor G1 (KLRG1) is an inhibitory receptor expressed on subsets of natural killer (NK) cells and T cells, for which no endogenous ligands are known. Here, we show that KLRG1 binds three of the classical cadherins (E-, N-, and R-), which are ubiquitously expressed in vertebrates and mediate cell-cell adhesion by homotypic or heterotypic interactions. By expression cloning using the mouse KLRG1 tetramer as a probe, we identified human E-cadherin as a xenogeneic ligand. We also identified a syngeneic interaction between mouse KLRG1 and mouse E-cadherin. Furthermore, we show that KLRG1 binds N- and R-cadherins. Finally, we demonstrate that E-cadherin binding of KLRG1 prevents the lysis of E-cadherin-expressing targets by KLRG1+ NK cells. These results suggest that KLRG1 ligation by E-, N-, or R-cadherins may regulate the cytotoxicity of killer cells to prevent damage to tissues expressing the cadherins.

B-1a cell origin of the murine B lymphoma line BCL1 characterized by surface markers and bacterial reactivity of its surface IgM.

B cells are divided into two categories: conventional or B-2B cells and B-1B cells, the latter of which are distinguished by their different ontogeny. B cell lymphoma 1 (BCL1), the first-reported case of a spontaneously developed mouse B-lymphoma, expresses CD5, surface IgM, Mac-1, CD43 and low level of B220, and is likely to have B-1a cell origin. However, antigens recognized by IgM produced by the BCL1 cells (BCL1-IgM) have not been identified. Here, we demonstrate that BCL1-IgM reacts with Escherichia coli (E. coli). Our initial finding that several recombinant proteins expressed in E. coli bound to BCL1-B20 prompted us to examine the possibility that BCL1 cells may bind E. coli. Indeed, BCL1 cells bound fluorescein-labeled E. coli. To elucidate the structure on the BCL1 cells responsible for E. coli-binding, we produced a monoclonal antibody capable of inhibiting BCL1 binding to E. coli. The antibody recognizes an idiotypic epitope on the BCL1-IgM. Moreover, polyclonal antibody against IgM and secreted BCL1-IgM purified from the supernatants inhibited BCL1 binding to E. coli. Finally, transfection of non-lymphoid cells with cDNA of heavy and light chains of BCL1-IgM conferred the cells ability to bind E. coli. These results clearly indicate that BCL1-IgM bind E. coli and suggest that BCL1 lymphoma is a typical B-1 cell-derived lymphoma, characterized not only by the surface phenotype, but also by the reactivity of its IgM with commensal bacteria.